Misc Physiological Responses to Space Flight

ASEN 5016 Lecture 10: Miscellaneous Physiological Responses to Space Flight

Objectives

1. Distinguish between major and minor physiological effects that occur during the various phases of space flight,

2. explain the associated time scales for each, and

3. identify their corresponding stressors

First, what exactly does ‘space flight’ imply?

Launch

Acceleration

· + Gx (eyeballs in) à up to 40 g’s survivable for ~10 seconds

· + Gy (eyeballs left)

· + Gz (eyeballs down) à just 2xg intolerable after ~12 minutes

Vibration

Varying natural frequency of body components

· Imperceptible to voluntary tolerance limits

· Biomechanical damage (e.g. ultrasonics)

Sound

85 dB à threshold of exposure without damage

120 dB à threshold of discomfort

· flight deck ~120 dB

· PLB ~ 160 dB

On-orbit

Minor – adaptations occur and reverse rapidly

Minutes to Hours…

· Height increase ~ 1-2 inches due to biphasic spinal response without structural loading

· Abdominal girth decrease

· Internal organ shift

· Posture altered (underwater, fetal position)

Breathing?

Lungs – remember hypothesis regarding blood and air distribution from top-bottom - gravity dependent or driven by lung geometry/function? Latter appears to be the case, but may also be indirectly affected by gravity…

Cephalic Fluid Shift – due to loss of hydrostatic pressure à facial puffiness, stuffy head, suppressed smell

-- actually starts on the pad…

On average, 60% of body mass is water

· intracellular fluid – in the cells

· blood plasma arteries and veins

· interstitial fluid – between tissues, cells

When standing (in 1g):

· Hydrostatic pressure increases BP by up to 100mm Hg at the feet

· BP drops correspondingly above the heart

· Related Symptoms - varicose veins and swollen feet

Major – more significant physiological effects with correspondingly longer recovery times

Hours to Days…

Vestibular system effects – fluid and otoliths are g-dependent and tied to other senses

· otoloths = static load and accel, semi-circular canals = accel, 3 d

· SMS – signal conflict vs. distortion, ‘asymmetry theory’

· So far, not predictable

Fluid loss – due to fluid shift (water volume in a 70 kg male is ~ 40 liters)

· extracellular fluid decreased ~15% by day 2

· ~3-4% total body water loss by day 4 or 5

Equilibrates to new baseline on orbit
Lower Body Negative Pressure (LBNP) use

Leg volume decrease – primarily due to fluid shift initially and later, muscle atrophy

Neuromuscular inhibition (skin pressure and proprioceptive senses)

· “Seat of the Pants” sensation lost

· Performance and local orientation impacts

Days to Weeks…

Blood plasma loss – due to fluid loss

average blood volume ~5 liters, of which, 3 liters = plasma, 2 = RBC mass

blood initially overloaded with RBC’s, production stops/slows, destruction increases

compare volume lost to blood donation of 1 pt (0.47 l), or ~10% of total blood volume (plasma + RBC)

· RBC mass reduced, perhaps even disproportionately wrt to plasma loss

· anemic-like?

· post-flight issues?

Muscle atrophy – use it or lose it

Cardiovascular deconditioning (reduced size)

· Altered Cardiac SV (increased) and HR (decreased)

· Result of decreased peripheral resistance?

Weeks to Months…

Bone atrophy - lost at rate of up to 2% per month from certain load bearing bones in the lower back and hips

· back pain

· fatigue

· fractures

· kidney stones?

Immunosuppression – dependency on bone marrow, psychological stress, weightlessness, radiation????

However, in flight exercise and presence of other stressors/variables complicate the study of (human) adaptation to weightlessness

Biomedical Countermeasures

Reentry (the ‘Gravitationally Challenged’ regime)

Fluid shift from blood to leg tissue à orthostatic intolerance

· Standing without moving can lead to fainting

· Pumping action elicited by one-way valves in veins in the legs.

CM – fluid load (saline) and LBNP

à leads to anemic-like condition following plasma replacement (RBC dilution)

Midodrine use

Loss of coordination – neurovestibular, proprioceptive feedback & hypersensitive reflex reactions

Cardiovascular reconditioning and baroreceptor reflex atrophy

Muscle and bone recovery

See research Areas at the National Space Biomedical Research Institute

Dynamic Interaction of Altered Functions and non-g Dependent Parameters

Weightlessness is primary driver of most of the physiological effects addressed in this class, but other factors can complicate the problem

Launch g’s and vibration – coupling effect? (shake and spin)

Closed environment issues and other Physical/Psychological factors

à Insomnia anxiety, depression, tension and interpersonal issues

à stress à physiological impacts

Workloads / Demanding Schedules

Radiation

Geomagnetic and Electrical fields, Hall Effect?

Other factors?